Electron emitting element



June 20, 1944. E. A. THURBER ELECTRON EMITTING ELEMENT Filed Dec. 18, 1941 m mp WW w? WA ATTORNEY Patented June 20, 1944 ELECTRON EMITTING ELEMENTI 7 Elmer A. Thurber, Brooklyn, N. YT, assignor to Bell Telephone- Laboratories, Incorporated, New York, N. Y., a corporation of New York I 7 Application December 18, 1941, Serial No. 423,409

9 Claims. 7 (Cl. 25027.5)

This invention relates to electron emitting elements for electron discharge devices and more particularly to methods of fabricating such elements.

For many-years thoriated tungsten filaments have served their usefulness in the thermionic arts as a source of copious electron emission in amplifiers, oscillators and other types of devices, but their adaptability has been impaired by two inherent disadvantages, one being the costly activation process in hydrocarbon gas for carbiding the tungsten and initiating the conversion of the thorium compound to active metallic thorium and the other being the resulting embrittlement of the tungsten core by the carbide produced during'this activation process. The latter fault predisposes'the filament to frequent and unpredictable'mechanical failures which in some cases, such as their use in airplaneindicators where they 'are subjected to unavoidable shock and vibration, cause sudden and unexpected interruptions in service which might prove disastrous.

Since carbonization is indispensible to the maintenanceof satisfactory emission by controlling production and distribution of metallic thorium inthis type of emitter, the only alternative would-bethe'production of an emitter of simulated characteristics without the decimating effect of the carbide.

An-object of this invention i to overcome, the difliculties usually attendant with thoriated tungsten emitter elements.

Anotherobject ofthe invention isthe production' of a composite'emitter having superior efficiency andrugged construction. a

' Aliurther object is to facilitate the activation of the emitter without inherent brittleness and carbide'formation in the core.

Still another object of the invention is to materially-decrease the thermal emissivity of the improved emitter while maintaining the operating current uniform and controllable. V a

In accordance with this invention, a composite emitter is produced which resembles or exceeds a thoriatedtungsten filament in electrical characteristics; emission and operating life, but which lacks the detrimental attribute of such a filament, by the application of a coating solution to a carrier core of tungsten wire or tubing having a wire or tubular helix of tungsten wound thereon to form a looking or retaining intermediate element between the core and the coating. The coating is sprayed or painted on the core and intermediate element and comprises a thorium compound mixed with a binder substance Which leaves no carbon residu upon heating the coat ing in vacuum, such as hexanitrated cellulose or gun-cotton dissolved in amylacetate. The om. pletely nitrated cellulose binder insures adequate adherence of the thoria coating to the core aridwhen the coating is flashed to partially reduce the material to active metallic thorium, the bind er is completely removed as gaseous products and no residual carbon remains in th coating 'or escapes into the core to cause embrittling action.

A feature of the invention relates to the prefiring of the powdered thorium compound to remove impurities, principally sulphur compounds,

which are poisonous to thermionic activity and cause shrinkage and poor adherence ofthe coat ing on the core. This is accomplished by been ing to approximately 1700 C. in a Wet hydrogen atmosphere for a predetermined interval, to remove impurities and especially sulphurous c'ontaminations so that a highly purified and sintered product is available for the coating mixture. These and other features of the invention will be more clearly understood from-the following: detailed description taken in connection with theg accompanying drawing:

Fig. 1 represents an electron discharge device in cross section, of the composite emitter em;

ployed in Fig. 1; and

Fig. 3 is a fragmentary view, partly in cross" section on an exaggerated scale, of the composite emitter constituting the subject of this invention.

Referring to Fig. 1, an ultra-high frequency; triode device is disclosed having an evacuated enclosing vessel formed of a dome-shaped shell I0 and a dish stem ll provided with conductors l2 sealed therein and projecting into the vessel;

Two of the conductors support a grid or'c'ontrol electrode l3 and a flanged anode I l, respectively,

in coaxial relation in an offset position with re spect to the axis of the vessel.

produced by sand, silicon carbide or" other abra sive. The other two conductors are provided with crook springs I 5 and I6 having their free endsin aligned relation along the axis ofthe grid'an'd anode and supporting therebetween-acomposite filamentary emitter ll which represents silhject-matter of this invention. r

The anode is formed of molybdenum having a blasted surface or a hollow tube, the latter structure being eco nomically advantageous in that the higher elec trical resistance materially reduces power con-ii sumption of such emitters when directly heated by an electric current.

The core and helix are coa tedfwithan electron."

ture is ready for use.

emitting bstance 20, for exa ple. a thorium compound, such as presintered and prepurifi'ed-x thorium dioxide, mixed with a binder material of acetate as a solvent. Other suitable solvents may be employed with the completely nitrated cellulose. The coating is applied preferably by spraying a mixture of thoria and binder over the helical covered portion of the core within the extremities of'the grid to a thickness to completely embed the helix iii. Since the turns of the helix are spaced on the core, the coating is securely affixed to the core and helix so that when the completed coating is fired, it is intimately locked between the turns and adheres to the core l8.

The commercial or chemically pure thoria has been found to produce poor adherence since it is quite light and fluffy and causes considerable shrinkage when a coating, is heated on a filament. core. It has also been found on chemical analysis that a typical sample of commercial thoria contains the following impurities.

' Per cent Thorium sulphate 7.52 Thorium nitrate 1- 0.06 Thorium chloride 0.05f Magnesium oxide 0.003 Silicon dioxide 0.'003 Iron; 0.006

and'traces of titanium, aluminum andtungsten.

The high sulphur impurities probably con,- tribute to poor adherence due to the evolution. of sulphurous gases during the operation of the emitter and the sulphur emanation also causes poisoning of the active emitting metal in the coating. 1

Inaccordance with this invention, the thoria is presintered and purified byfla thorough firing at 1700 C. in a wet hydrogen atmosphere or hy-.

in the binder carrier or fluid. Thistreatment materially reduces the impurities and especially the sulphur content from a high value of 7.52 to as low as. .007. This represents a reduction to 1/ 1000 of. original value and the purified thoria insures higher emission values inthe cathode.

The binder is usually added as a 12.5 per cent solution by weight, of the completely nitrated cellulose or gun-cotton (nitrogen content 13.4 per cent) in amyl acetate. The constituent proportions of a typical spraying mixture are Prepurified thorium oxide grams 500 Gun-cotton solution do 75 Amyl acetate "cubic centimeters 575 This mixture is placed in a ball-mill jar containing 500 grams of pebbles and run in a roller continuously for 48 hours after which the mixa This procedure produces a homogeneous mixture with the solid particles finely divided and well dispersed in the suspendj ing medium.

' The mixture is sprayed onto the composite core as indicatedabove and after preliminary mounted in the device as shown in Fig. 1.

drying in air the composite coated emitter is The completed device is then baked on the pumpstation at 400 C. and the anodeis outgassed at 1000 to1050 C. by high-frequency current for two minutes with the filament incandesced at 1.3 ampere currents. After the removal of the gases by the pumps and a high vacuum is attained, the filament is flashed at 2.0 amperes for one half minute with the high-frequency reduced to give about900 C. anode temperature. The filament current is reduced to the operating point of 1.3 amperes and when the pressure attains the value of 1 10 millimetersof mercury, the device is sealed off the station.

During the flashing treatment of the thoria coating, the high temperatures cause the evolution of water vapor and carbonaceous gases spontaneously due to the reaction between the nitrogen, carbon, hydrogen and oxygen so that the. binder material is completely removedas 100 per cent gaseous products and the thoria is partially reduced to active metallic thorium. The resultant coating is carbon-free sincethere is more than sufficient oxygen in the hexanitrated cellulose to completely burn out all thecarbon so that no carbon is diffused into the coreor intermediate helix and the coating matrix to depreciate the emissive properties of the coating or cause embrittling of the core material. The thermal emissivity of the coating matrixis substantially constant over a long, periodof'operating life and the total emission values are ex.-.

ceedingly high, being of the order .of 100 mils per Watt or 3 amperes per square centimeter.

The filament processed in accordance with this invention has a low constant thermal emissivity,

' and therefore a low operating current, thus matoa considerable extent. In tubes where abnormaliy high activity. is paramount the higher thermal operating point. may be employed with somewhat shorter butsatisfactory life due to reduced evaporation and improved adherence of thematrix coating of thoria presintered according to this invention.

It is, of, coursaunderstood that the invention is,

not restricted to the specific construction set forth except in certain aspects with respect to the prefired emitting material and the completely nitrated cellulose binder ingredients. Therefore, the invention is to be construed by the scope of the appended claims.

What is claimed is:

1. An electron emitter comprising a metallic core, and a coating matrix thereon consisting of purified electron emitting material and a binder including completely nitrated cellulose.

2. An electron emitter comprising a metallic core, and a coating matrix thereon consisting of purified and sintered thoria suspended in a binder fluid including completely nitrated cellulose.

3. An electron emitter comprising a metallic core, and a coating matrix uniformly deposited thereon consisting of purified and prefired thoria, free from carbon residue.

4. An electron emitter comprising a metallic core, a helical winding thereon having spaced turns in contact with said core, a carbon-free thoria matrix uniformly applied on said core, and said helical winding embedded therein, said thoria being partially reduced on the surface to serve as a source of electron emission.

5. An electron emitter comprising a tungsten core, and a prefired matrix coating thereon of thorium oxide applied with a binder of cellulose having a formula Of C12H14(ONO2) 604.

6. A coating suspension for electron emitters, comprising a highly refractory prefired emitting substance, completely nitrated cellulose, and a solvent therefor.

7. A coating suspension for electron emitters, comprising prefired and presintered thorium oxide, hexanitrated cellulose solution, and amyl acetate solvent in the proportions of 500 grams of the oxide to grams of solution in 575 cubic centimeters of solvent.

8. The method of activating a composite emitter having a highly refractory material coated on a conducting core which comprises, applying said material suspended in a binder solution of hexanitrated cellulose dissolved in a solvent to a core to form a matrix, heating said matrix to a reaction temperature to spontaneously combine the excess nitrate content with the carbon of .said cellulose, removing the resultant gaseous products, and reducing some of the refractory material to form emissive metal.

9. The method of activating a composite emitter having thorium oxide coated on a tungsten core which comprises, applying said thorium oxide suspended in a. binder containing a cellulose material having. the formula of C12H14(ONO2) 604 dissolved in amyl acetate to said core to form a matrix coating, heating said matrix to a reaction temperature to spontaneously combine the nitrate content with the carbon of said cellulose, removing the resultant gaseous products, and reducing some of the thorium oxide to metallic thorium.

ELMER A. THURBER. 

